Group III-nitride semiconductors (e.g. GaN, AlN, InN, InGaN, AlGaN, and InAlGaN) are important for the fabrication of a variety of semiconductor devices, such as UV, blue and green light emitting diodes (LEDs) and laser diodes (LDs), high frequency devices (e.g. high electron mobility transistors, also known as HEMTs), high power switching devices, UV detectors, etc. Group III-nitride-based LEDs hold promise for general illumination applications because of their energy savings potential, long lifetime, compactness, and high efficiency. Despite the advances made in recent years in efficacies of commercial group III-nitride-based blue, green and white LEDs, LED lighting products still fall short of key performance and price requirements needed to meet the demands of the general lighting market. Group III-nitride-based LEDs emitting blue, green and UV light are usually produced with group III-nitride thin films grown on a substrate. To date, native group III-nitride substrates (GaN and AlN) are either not commercially available in large sizes and/or too costly to be considered as a viable choice of substrates for commercial volume production of LEDs. Commercial group III-V nitride-based HB-LEDs (high brightness—LEDs) are currently fabricated from thin films grown heteroepitaxially on sapphire substrates (which at present account for the majority of group III-nitride LEDs produced in the world) and SiC substrates using a MOCVD (metalorganic chemical vapor deposition) technique (also known as MOVPE technique). Other commercially available substrates, such as Si, GaAs, ZnO, other oxides (e.g. LiAlO3) and even metals were studied for group III-nitride epitaxy and LEDs with little success.
Despite a low cost and a wide commercial availability, the incumbent sapphire substrates (as well as SiC substrates) have several drawbacks that limit the ability of commercial group III-nitride LED manufacturers to achieve a lower cost and a better LED performance. Theses drawbacks include: a large mismatch of thermal expansion to GaN; a high mechanical hardness or strength; and a high chemical inertness.
Sapphire is known to have a coefficient of thermal expansion (CTE) much larger than that of GaN and other group III-nitride alloys (InGaN and AlGaN). Therefore, a sapphire substrate with group III-nitride LED layers grown on it at a high temperature (e.g., about 1050° C.) will have a significant bow when cooled to room temperature due to thermal expansion mismatch. The bow complicates the subsequent LED device fabrication and test processes. In fact, as the substrate size gets larger (e.g. 10 centimeter (cm) (4-inch) or larger), the bow due to CTE mismatch between sapphire and GaN gets larger. The bow is an issue resulting in difficulties of implementing large diameter substrates in commercial LED production. Without being able to scale up the LED manufacture process to adopt progressively larger substrates (10 centimeter (cm) (4-inch), 15 cm (6-inch), 20 cm (8-inch), etc.), the cost of group III-nitride LEDs will remain too high for general illumination applications.
Since sapphire also has a high hardness (twice as that of GaN), coupled with a large CTE mismatch with GaN, a significant stress develops at the interface between the GaN epilayer and the substrate during the cool-down process after a MOVPE growth, and such a stress can lead to additional lattice defects and even cracks, resulting in a low LED performance and a low LED yield. In addition, because of the high hardness, the fabrication of sapphire substrates from sapphire crystal boules (i.e. coring/cropping, slicing, polishing and chemical-mechanical polishing (CMP)) is a relatively high cost operation compared to that of Si and GaAs substrates.
The high chemical inertness, or stability, of sapphire substrates is beneficial because sapphire can withstand the chemical etching during group III-nitride epitaxy. However, the high inertness of sapphire, coupled with a high mechanical hardness, does inhibit an easy removal of the substrates from the III-nitride layer during fabrication of LED dies. Complete removal of substrates to create “thin-film” LEDs is highly desirable because a “thin-film” LED has better light extraction (leading to a better efficacy), better heat dissipation (leading to a longer lifetime as well as a better efficacy), and a simpler LED device structure (leading to a lower cost for device processing). The laser lift-off technique for removing sapphire substrates from a group III-nitride device appears to be too costly for commercial volume production of LEDs. Therefore, an alternative substrate material that can address the above issues associated with the incumbent sapphire substrates and SiC substrates is needed for the commercial success of group III-nitride-based LEDs for solid state lighting applications. Such a substrate material will also benefit development and commercialization of other group III-nitride-based semiconductor devices.
The above-mentioned drawbacks of sapphire and SiC substrates also hinder the realization of low-cost, large-diameter (>5 centimeters) (>2 inches), crack-free, and freestanding GaN thick substrates (20 to 1000 micrometers (μm) in thickness) or bulk crystal boules (with a thickness larger than 1 mm) that can be grown via a hydride vapor phase epitaxy (HVPE) technique. The availability of low-cost freestanding GaN substrates or bulk substrates in high volume is beneficial to the development and commercialization of the above-described group III-nitride-based semiconductor devices.